1. Field of the Invention
The invention relates to a sigma-delta modulator, and more particularly, to a continuous-time sigma-delta modulator (CT-SDM) with an additional feed-forward path specified for the high frequency signal components.
2. Description of the Related Art
For continuous-time sigma-delta modulators with high bandwidth and high sampling frequencies, the stability is the major design challenge. In general, compared with continuous-time sigma-delta modulators of feed-back types, continuous-time sigma-delta modulators with feed-forward topology save more power and area by replacing multiple feed-back digital-to-analog converters with feed-forward paths. A summing amplifier is necessary for a conventional continuous-time sigma-delta modulator with feed-forward topology. However, the configuration of the summing amplifier induces additional power. Thus, continuous-time sigma-delta modulators with AC-coupling or passive summing methods are provided to eliminate the above summing amplifier. Unfortunately, there are some drawbacks occurred in the continuous-time sigma-delta modulators with AC-coupling or passive summing methods, particularly for high sampling frequency prototypes: low stability, less driving capability for driving quantizers, and/or large power consumption.